Mercury-arc lamp

ABSTRACT

A mercury-arc lamp formed of a generally cylindrical-shaped tube with electrodes mounted at either end thereof and comprising a line source of light when illuminated from the central portion of the tube between the electrodes and with the tube enlarged adjacent the electrodes so as to reduce devitrification of the tube and provide a long-life mercury-arc lamp.

United States Patent [72] lni/entors Jlrobegnwlt Chlhmken; OsnmuTnkeuchl, Tokyo, both of, Japan [21] AppLNd 836,871 [22] FiledJune26,1969 [45] Patented lul 13.15371 [73] Asslgnee Bony Corperitlnnl'okymhpun [32] Priority June29,1968 [33] Japan [311 43/45376 [54]MERCURY-ARC LAMP 3 Claims, 6 Drawing Figs.

[52] U.S.Cl 313/220,

313/l84,313/204,313/217 [51] lnt.Cl H0lj6l/30 [S0] FieldofSearch313/184,

[561 References Cited UNITED STATES PATENTS 2,094,694 10/1937BeletalmmuuuWm. 315/112 2,321,178 6/1943 Beurneetel.mmmum 313/204XFOREIGN PATENTS 107,110 4/1939 Australla..............l 313/217 PrimaryExaminer-Raymond F. Hossfeld Attorney-Hill, Sherman Meroni, Gross andSimpson ABSTRACT: A mercury-arc lamp formed ofa generallycylindrical-shaped tube with electrodes mounted at either end thereofand comprising a line source oflight when illuminated from the centralportion of the tube between the electrodes and with the tube enlargedadjacent: the electrodes so as to reduce devitrification of the tube andprovide a long-life mercury-arc lamp.

PATENTEDJULI 3:971 3.593.056

SHEET 1 [IF 2 Pm? Aer J IRO DEGAWA M-T EUHI BY U awn/WSW MERCURY-ARCLAMP BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to a mercury-arc lamp and particularly to amercury-arc lamp which provides a source of fineline radiation with ahigh degree of brightness.

2. Description of the Prior Art In television color picturetubes of theaperture grill or Chromatron (registered trademark) type which areformed of many parallel grid elements constructed of metal wires orstrips which extend across the face of the tube at predeterminedintervals and are mounted adjacent the color phosphor screen, anelectron beam is directed to the screen through the grid elements toexcite the phosphors to obtain particular colors.

The phosphor screen of the color picture tube of these types are made upof a plurality of phosphor strips that emit red, green and blueemissions. The strips are sequentially. arranged in a repeating cyclicorder to obtain the desired color combinations.

The present preferred method for making such phosphor screens comprisesplacing a grid deviceon a face plate which has been coated over theentire interior surface with a phosphor slurry of a particular coloremissive phosphor and a photosensitive material and the coated interiorsurface is then exposed to radiation by light from a line source. Theexposed photosensitive material hardens and the unexposed photosensitivematerial is removed so as to leave the phosphor slurry only at thoseareas which have been exposed. It has been dif ficult to reduce the tubediameterof the mercury-arc. lamp so as to obtain high resolution andthelightemitted from such prior art lamps has been fairly wide and themercury-arc lamps have not been satisfactory as a line source of light.Attempts have been made to use optical lens for reducing the prior artlamps to a line source but this .is difficult and a substantial portionof the light is lost due to the lens.

Although the luminous portion in mercury-arc lamps may be renderedextremely fine by reducing the diameter of plasma produced between theelectrodes, which can be achieved by reducing the tube diameter of thelamp, this results in a disadvantage in that the reduced tube diametercauses devitrification of the tube envelope clue to heat by radiationwhich shortens the life of the lamp.

SUMMARY OF THE INVENTION The present invention comprises a mercury-arclamp which ha. tube diameter which is reduced in the central portion toprovide a line source of illumination and which has enlarged sphericalcavities formed in the tube at either end of the active portion adjacentthe electrodes of the lamp so as to avoid devitrification of the tubeenvelope and to provide fine and linear radiation of high intensity thatmay be used to [make phosphor screens of color picture tubes'withoutusingta lens system.

Accordingly, one object of this invention is to provide a mercury-arclamp of high luminance which provides fine and linear radiation.

Another object of this invention is to provide a mercury-arc lamp whichis not subject to devitrification of the tube envelope and is longlasting.

Still another object of the invention is to provide a mercuryarc lampwhich is suitable for use as a line source of light in making phosphorscreens of color picture tubes without the need of a lens system.

Other objects, features and advantages of the invention will be readilyapparent from the following description of certain preferred embodimentsthereof taken in conjunction with the accompanying drawings, althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts of the disclosure, and in which:

FIG. I is a schematic view illustrating the optical printing method formaking a color phosphor screen;

FIG. 2 is a cross-sectional view of a prior art mercury-arc lamp;

FIG. 3 is an enlarged cross-sectional view illustrating one example of amercury-arc lamp according to this invention; and

FIGS. 4, 5 and 6 are graphs illustrating the degree of devitrificationof the tube envelope and are utilized for explaining the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates apparatusfor exposing a phosphor screen for constructing a color television tube,for example. A phosphor slurry 2 is coated on the entire inside surfaceof .a panel 1 of the tube envelope of a color picture tube upon which itis desired to form a color phosphor screen. The phosphor 2 might beformed, for example, of a red color emissive phosphor and aphotosensitive binder. An optical mask 3 which has an optical patterncorresponding to the pattern the desired red phosphor strips of thecolor phosphor screen which will be ultimately formed is placed betweenthe panel 1 and a light source 4 so as to expose the red phosphor stripsat the desired locations. The phosphor slurry 2 is placed on the insidesurface of the panel 1 and is exposed through the optical mask 3 toirradiation from the light source 4 to form in the slurry a latent imageof the optical pattern of the mask 3. Then the interior surface of thepanel 1 is subjected to a developing process to obtain red colorphosphor strips of a predetermined pattern. The unexposed phosphorbetween the exposed red color phosphor strips is removed and the processis repeated to place green and blue color emissive phosphors on thepanel The light source 4 is usually a mercury-arc lamp. It is desiredthat the light source 4 be a linear light source which extends in thelongitudinal direction of the phosphor'strips that will 'be ultimatelyformed so as to insure uniform exposure of the phosphor strips to lightthroughout their entire lengths.

However, prior mercury-arc lamps have not been bright enoughtosatisfactorily serve as linear light sources and an optical system hasbeen required. It is difficult to produce a linear light beam using alens system and also the use of a lens system causes a substantial lossof the light energy.

FIG. 2 illustrates a prior art mercury-arc lamp. A pair of electrodes 7Aand 78 project into a tube 6 made of quartz glass from either end in theaxial direction. Mercury holes 8A and 8B which are filled with mercury 9are formed at both ends of the tube 6 and an inert gas such as argon,xenon or the like is sealed in the tube 6. It is to be particularlynoted that in prior art mercury-arc lamps the inner diameter of theitube6 is small at the inner ends of the mercury holes 8A and 88 as indicatedby the numerals 10A and 10B so as to prevent the mercury 9 from flowingout through the holes 8A and"-8B. The inner diameter of the tube 6 issubstantially uniform between the ends of the electrodes 7A and 7B.

So as to render the luminous portion of the mercury-arc lamp linear, thediameter of plasma produced between the electrodes 7A and 7B is madesmall by reducing the diameter of the tube 6. However, this causesdevitrification of the tube 6 which results in a loss in the amountof'light produced and -the devitrification of the tube 6 has started itprogresses.

rapidly and spreads. In some cases, this introduces heat distortion inthe glass tube and decreases the pressure which the glass can withstandresulting in explosion on the tube 6.

When the inner diameter of the tube 6 is sm,all,,the inside surface ofthe tube 6 is very close to the electrodes 7A and 7B.

This allows negative ions emanating from the electrodes to impinge uponthe inner walls ofthe tube with great energy and the electrodes areheated above l000 or more. Such intense heat causes the closely spacedglass tube to devitrify as described above resulting in short servicelife of the mercuryarc lamp. When the quartz glass is heated to a hightemperature it becomes conductive and creeping discharges occurresulting in a loss of energy. The creeping discharge is directed to thepositive electrode but when the mercury-arc lamp is ignited by an ACcurrent, the current reverses in polarity and the creeping discharge isdirected to the center between the electrodes of the tube 6. In the caseof lighting with AC current, the creeping discharge is intenselyproduced and loss in energy is as much as several hundred times thatwhich occurs during lighting by DC current. Also, once the creepingdischarge has started, it absorbs heat generated by the plasma anddevitrification of the tube is increased. This also causes a localincrease in the vapor pressure of the tube, thus lowering the luminousefficiency of the lamp and increasing the likelihood of explosion of thetube.

The present invention is illustrated in Figure 3. A substantiallycylindrical tube 11 is constructed of quartz glass and rodlikeelectrodes 12A and 12B partially extend into the tube from both endsthereofin the axial direction.

The ends 120 and 12b of the electrodes 12A and 12B are formed in theform of truncated cones with the base of the cones having a diameterofabout 0.5 mm. and the point ofthe electrodes are tapered to a diameterof about 0.3 mm., respectively. The distance between the electrodes 12Aand 12B may be approximately 15 mm.

The tube of this invention is formed such that the inner diameter of thetube 11 is within the range from 0.5 to 1.4 mm. which is much smallerthan that of conventional mercuryarc lamps. Also, the inner walls of thetubes adjacent the end portions 120 and 12b of the electrodes 12A and12B are enlarged to form spherical cavities 14A and 148. The diametersof the cavities 14A and 148 may be selected such that the distancebetween the inner wall of the cavity and the end of the electrode isgreater than the diameter of the tube in the central portion. When thediameters of the inner diameter of spherical cavities 14A and 14B hasbeen in the range of 2 to 2.5 mm., very satisfactory results have beenobtained. Such diameters provide a substantial spacing between theactive ends of electrodes 12A and 12B and the inner walls of sphericalcavities 14A and 14B and substantially inhibit devitrification.

Narrow portions of the tube adjacent the cavities 14A and 14B are filledwith mercury 15. The portions of the tube adjacent the ends are formedof quartz glass, gradiant seal glass 16 as shown and tungsten glass 17through which the electrodes 12A and 12B pass.

FIGS. 4, and 6 are graphs illustrating the degree of devitrification ofthe tube 11. In these curves the particular tube of the invention testedhad an outer diameter of 4 mm. and an inner diameter of I mm. Thedistance between the electrodes 12A and 128 was l5 mm. The diameters ofthe electrodes 12A and 12B were 0.5 mm. at the base and 0.3 mm. at thesmall ends, respectively.

The degree of devitrification is plotted on the basis that indicatesthat the tube is opaque to a degree such that an electrode in eachcavity cannot be seen from the outside of the tube.

FIG. 4 illustrates the degree of devitrification of the tube at thecavities 14A and 148 relative to the lighted time of the mercury-arclamp when the pressure P of cooling air fed to the lamp was l.5 kg./cm.the diameter d of the cavities 14A and 148 being used as a parameter.For example, it is to be noted that the top curve in FIG. 4 which islabeled d==l.0 mm. has a devitrification degree much higher thandiameters of d=l.5 mm., d=2.0 mm. or d=2.5 mm.

FIGS. 5 and 6 show, respectively, the degree of devitrification of thecavities and luminous portion of the tube as a function of the diametersd of the cavities 14A and 14B of the tube 11 with the pressure P of thecooling air being used as a parameter.

The graphs show that the devitrification degree decreases with anincrease in the distance between each electrode to the inner wall of thetube but when d=2.5 mm. or more, the improvement of the devitrificationdegree does not substantially increase with increase of the distance d.An increase in the diameter of the cavities causes an increase in thepressure within the cavities resulting in less mechanical strength ofthe tube. When d is equal to or less than 2 mm. the devitrificationdegree is large. Therefore, it is desired that the diameter of thecavities 14A and 148 be in the range ofabout 2 to 2.5 mm.

The diameter of the main portion of the tube 11 between the electrodeswith the exception of the cavities 14A and 14B is selected to be in therange between 0.5 to 1.4 mm. because tubes having diameters ofless than0.5 mm. are low in working efficiency whereas tubes with diametersexceeding 1.4 mm. are not preferred because the diameter of the plasmais increased to such an extent that the lamp is inadequate as a linesource of light and also the surface tension of the mercury 15 isexceeded which allows the mercury to flow from the mercury holes whenthe lamp is mounted in a vertical direction, The tube according to thisinvention is selected so that it is small and linear irradiation can beproduced. Therefore, the use of the mercury-arc lamp of this inventionas the light source for optical printing of the color phosphor screendoes not need the optical system 5 illustrated in FIG. 1. As a matter offact, it has been discovered that the mercury-arc lamp of this inventionincreases the brightness five to 20 times over that of conventionalmercury-arc lamps such as illustrated in FIG. 2 with conventionaloptical systems.

Also the mercury-arc tube of the invention comprises a tube with a smalldiameter which has small overall area which decreases the overallpressure on the tube and allows it to withstand increased pressure. Thisincreases the mercury vapor pressure and enhances the luminousefficiency of the lamp.

Also, since the electrodes are spaced a substantial distance from theinner wall of the tube due to the cavities 14A and 14B surrounding them,the probability of impingement of negative ions from the electrodes 12Aand 12B upon the inner wall of the tube can be decreased and the energyof any ions impinging upon the wall of the tube will also be reduced.

In addition, due to the spacing of the electrode from the inner wall,the so-called thermal layer is provided in the space which reduces thetransmission of heat from the electrode to the wall of the tube so thatdevitrification of the tube can be effectively prevented and the servicelife of the lamp will be substantially increased.

The tapering of the ends of the electrodes as illustrated at 12a and12b, reduces the transmission of heat from the heated electrodes andheat radiation is low. Therefore, the electrodes can be maintained at ahigh temperature and an electric charge concentrated on the electrodesto insure efficient discharge between the electrodes.

It will be apparent that many modifications and variations may beeffected without departing from the scope of the novel concepts of thepresent invention.

We claim as our invention:

1. A mercury-arc lamp comprising:

a tube made of glass;

two electrodes sealed in the tube at opposite ends thereof;

mercury contained in the tube at both ends thereof; an inert gas sealedin said lamp;

enlarged spherical cavities formed in the tube about the ends of saidtwo electrodes; the inner diameter of the main the range ofO.5 to L4mm.; the inner diameters of said spherical cavities being greater thanl.5 mm.; and said two electrodes being circular in cross section andhaving diameters of about 0.5 mm.

2. A mercury-arc lamp according to claim 1 wherein said twoelectrodesare formed with truncated tapered ends.

3. A mercury-arc lamp according to claim 2 wherein the inner ends ofsaidtwo electrodes are about 0.3 mm.

portion of the tube being in

2. A mercury-arc lamp according to claim 1 wherein said two electrodesare formed with truncated tapered ends.
 3. A mercury-arc lamp accordingto claim 2 wherein the inner ends of said two electrodes are about 0.3mm.